The present invention relates to a transmission control device for controlling the operation of a transmission system having a main transmission and a sub transmission.
A transmission system for vehicles is known which has an automatic or manual main transmission capable of transmitting the power of the engine while changing the speed and a sub transmission which is connected to the output side of the main transmission and capable of transmitting the power to the vehicle wheels in different speed modes. The different speed modes of the sub transmission is attained, for example, by a planetary gear type mechanism which has a sun gear, planetary pinions, a planetary pinion carrier and a ring gear, the sun gear and the carrier being connected to the input shaft and the output shaft of the sub transmission, respectively. In operation, the sun gear and the carrier are brought into and out of engagement with each other through a clutch, while the ring gear is adapted to be releasably fixed by a clutch, so that a direct-coupling drive mode which provides a high output speed is obtained through engaging the clutch and releasing the brake, while a reduced-speed drive mode is obtained through disengaging the clutch and putting the brake into effect. The selective operation of the clutch and brake is conducted hydraulically by means of respective servomotors which in turn operate by low-high change-over valves operative in accordance with the position of a manual shift valve and oil pressure corresponding to the vehicle speed. Each of the change-over valve has a spool which receives at its one end a governor oil pressure correspnding to the speed of the output shaft of the main transmission so that, when the driver has operated the sub transmission for down-shift from the direct-coupling drive mode to the reduced-speed drive mode, the spool operates in response to the pressure corresponding to the speed of the output shaft of the main transmission so as to prevent the change-over of the sub transmission to the reduced-speed mode, thereby avoiding troubles such as over-revolution of the engine which otherwise may be caused by inadequate down-shift operation.
Thus, the known transmission system of the type described has encountered a problem in that, since the spool of the low-high change-over valve is operated by the governor pressure corresponding to the speed of the output shaft of the main transmission, the speed changing characteristics of the transmission system could not be optimized for the actual state of running of the vehicle.